In the prior art, there has been a continuing effort to develop radar systems which are suitable for high-resolution applications, such as ground-mapping and air reconnaissance. Initially, this finer resolution was achieved by the application of pulse-compression techniques to conventional radar systems which were designed to achieve range resolution by the radiation of a short pulse, and angular, or azimuth, resolution by the radiation of a narrow beam. The pulse-compression techniques provided significant improvement in the range resolution of the conventional radar systems, but fine angular resolution by the radiation of a narrow beam still required a large-diameter antenna which was impractical to transport with any significant degree of mobility. Subsequent to the development of pulse-compression techniques, synthetic aperture radar (SAR) techniques were developed for improving the angular resolution of a radar system to a value significantly finer than that directly achievable with a radiated beam width from a conventional antenna of comparable diameter.
In the prior art, an equivalent to a large-diameter antenna was established which was comprised of a physically long array of antennas, each having a relatively small diameter. In the case of a long antenna array, a number of radiating elements were positioned at sampling points along a straight line and transmission signals were simultaneously fed to each element of the array. The elements were interconnected such that simultaneously received signals were vectorially added to exploit the interference between the signals received by the various elements to provide an effective radiation pattern which was equivalent to the radiation pattern of a single element multiplied by an array factor. That is, the product of a single element radiation pattern and the array factor resulted in an effective antenna pattern having significantly sharper antenna pattern lobes than the antenna pattern of the single element.
SAR systems are based upon the synthesis of an effectively long antenna array by signal processing means rather than by the use of a physically long antenna array. With an SAR, it is possible to generate a synthetic antenna many times longer than any physically large antenna that could be conveniently transported. As a result, for an antenna of given physical dimensions, the SAR will have an effective antenna beam width that is many times narrower than the beam width which is attainable with a conventional radar. In most applications of SARs, a single radiating element is translated along a trajectory, to take up sequential sampling positions. At each of these sampling points, a signal is transmitted and the amplitude and the phase of the radar signals received in response to that transmission are stored. After the radiating element has traversed a distance substantially equivalent to the length of the synthetic array, the signals in storage are somewhat similar to the signals that would have been received by the elements of an actual linear array antenna.
An SAR can obtain greater resolution than a conventional linear array of equivalent length as a consequence of the noncoherent transmission from the sampling points of the SAR. The stored SAR signals are subjected to an operation which corresponds to that used in forming the effective antenna pattern of a physical linear array. That is, the signals are added vectorially, so that the resulting output of the SAR is substantially the same as could be achieved with the use of a physically long, linear antenna array.
In generating the synthetic antenna, the signal processing equipment of an SAR operates on a basic assumption that the radar platform travels along a straight line trajectory at a constant speed. In practice, an aircraft carrying the radar antenna is subject to deviations from such nonaccelerated flight. It is therefore necessary to provide compensation for these perturbations to straight-line motion. This motion compensation must be capable of detecting the deviation of the radar platform path from a true linear path.
Briefly, and referring now to FIG. 1 in the drawings, an SAR system carried by an aircraft 10 maps a target region 12 by transmitting and receiving radar signals at various sampling points S.sub.1, . . . , S.sub.N, along the flight path 14 of the aircraft. The SAR system may be positioned in the nose portion 16 of the aircraft 10, for example (see FIG. 2). The SAR will typically require an inertial navigation system (INS) 18 for calculating the position of the antenna 22 at the sampling points S.sub.i. However, in addition to the problems discussed above, the antenna 22 may be physically separated from the INS 18 by a physical support, such as a lever arm 20, for example. The flexible modes of the lever arm cause the data indicating the position of the antenna 22 to be imprecise. Thus, the antenna 22 may deviate from the position as reported by the INS 18. In other words, and referring now to FIG. 2, at every sampling point S.sub.i in the flight path 14, the antenna 22 may be displaced slightly to a position denoted S.sub.i '. This is referred to as residual antenna motion. If not compensated for, residual antenna motion will corrupt the signal phase, possibly to the extent that the resulting degraded image is of no practical use. The severity of the problem increases with the desired resolution.
The residual motion can be alleviated (at least in the frequency region where the energy of the disturbance concentrates) by stiffening the antenna lever arm sufficiently. However, this would add to the weight and the cost of the SAR system. Moreover, such a solution would not cover other sources of residual motion, e.g., flexible modes in the antenna itself or slow INS report rates.
An accelerometer or a full INS on the antenna itself would provide more accurate readings of the antenna position. This, however, would add extra expense, and the resolution of the SAR would still be limited by the accuracy and/or report rates of the INS. In addition, flexible modes in the antenna would still produce errors.
By analyzing the received signal, in some situations certain types of residual motion can be removed. These signal analysis techniques are generally known as autofocusing. One technique, feature referenced autofocusing, arises from the fact that if subapertures are formed from different periods of the integration time, the corresponding images have position shifts corresponding to the almost constant rates of phase change across the subapertures. These nearly linear phase histories can then be pieced together to yield an estimate of the residual motion. This method, however, works well only if the residual motion is of low frequency; it is mainly used to remove second order residual motions.
Another technique is to correct the errors by using dominant point targets. In this technique, the main antenna beam is first divided into subbeams. If there is a dominant scatterer in a certain range cell within a subbeam, the corresponding time signal will have a linear phase on which is superimposed the residual motion. The latter can thus be estimated as deviations from the linear phase. The estimates corresponding to the various such dominant scatterers are then averaged to provide a single estimate of the residual motion. This technique works well if: (i) there is a reasonable number of dominant scatterers in the scene and (ii) the residual motion does not contain frequencies so high that energy from dominant scatterers leaks across subbeams. When the dynamic range of the scene is small (as would often be the case when no man-made objects are present or when attempt is made to lower the transmission power), the condition (i) may not be satisfied with large subbeams. Decreasing the size of subbeams would likely violate condition (ii).
The technique described above can be simplified, resulting in the so-called radio camera algorithm. This algorithm isolates those range lines which are composed of a point reflector dominating the whole range line. This technique suffers the same limitations as that described above, only to a greater extent.
A further technique, described in U.S. Pat. No. 4,617,567, is termed a tuned autocompensator. The tuned autocompensator has been proposed to examine range lines for symmetric triplets of apparent reflectors situated at a distance from one another corresponding to some characteristic frequency of the residual motion. Such a triplet would come from the main antenna lobe and first order side lobes arising from a point reflector. By processing the triplets, estimates of magnitude and phase of the residual motion at a characteristic frequency can be obtained. This technique performs well when magnitude of the residual motion is less than a fraction of the carrier wavelength, but not otherwise.
The present invention provides a method for compensating for antenna residual motion in an SAR system. The described invention will use signal processing means to estimate the residual antenna motion from the received signal and subsequently remove its effect on the image. This will be referred to as an autocompensator.
A novel idea of this invention is that when the magnitude of a sinusoidal residual motion exceeds a fraction of the carrier wavelength, not only the fist order side lobes but also the second order, and, in rare situations, the third order side lobes are necessary for the determination of the residual motion magnitude and phase. Moreover, along with the main antenna lobe, the side lobes of order up to three are always sufficient for that purpose.